ONTOGENY, SEX DIMORPHISM, AND NEONATAL SEX-HORMONE DETERMINATION OF SYNAPSE-ASSOCIATED MESSENGER-RNAS IN RAT-BRAIN

Sex hormones influence neurite outgrowth and synaptogenesis in certain hormone-dependent areas of the rat brain during neonatal development. These alterations are thought to mediate changes in brain structure a nd function between the sexes. Growth-associated protein 43 kDa (GAP-4 3) gene expression is estrogen-regulated in the adult ventromedial hyp othalamus (VMH) and sexually dimorphic (M:F = 1.8:1) in adult cortex ( CTX). Such effects intimate hormonal regulation of synaptic plasticity . To investigate the nature of these dimorphisms, the present study ex amined the ontogeny of expression of mRNAs encoding 3 neural-specific proteins: GAP-43, SCG10, and synaptosomal-associated protein 25 kDa (S NAP-25); and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), in the VMH and CTX; and also the effects of altering the neonatal sex hormona l milieu on the development of these adult dimorphisms. Levels of spec ific mRNAs in VMH and CTX were quantitated by slot-blot hybridization in rats of both sexes at different postnatal ages. To determine the in volvement of neonatal sex hormones on the levels of these mRNAs, male neonatal rat pups were treated with an estrogen receptor antagonist or an aromatase inhibitor, and neonatal female pups were treated with te stosterone or estrogen prior to slot-blot evaluations in adulthood. In VMH, GAP-43 mRNA levels were high on days P1 and P4 with a 3-fold dec rease by day P23; in CTX, GAP-43 mRNA first increased by day P11, then fell to baseline by day P23. In VMH, SCG10 mRNA showed only small inc reases with time; but in CTX, there was a 5-fold drop from days P4 to P23. In VMH, SNAP-25 mRNA was low and changed only slightly; but in CT X there was a 5-fold increase between days P4 and P60. At birth, there was no sex dimorphism in either VMH or CTX, but the levels of all 3 n eural-specific mRNAs were sexually dimorphic in adult CTX (M:F = 1.76 for GAP-43, 1.46 for SCG 10, 1.44 for SNAP-25). GAPDH mRNA levels were regulated developmentally in VMH and CTX, but there was no sex dimorp hism in either area. In male rats who received either an estrogen anta gonist or aromatase inhibitor at birth, the CTX GAP-43 and SNAP-25 mRN A levels fell by 30%, to levels similar to untreated females. Converse ly, in female rats, neonatal treatment with either testosterone or est rogen increased GAP-43 and SNAP-25 mRNA levels by about 30%, to levels similar to the untreated adult male. SCG10 levels did not demonstrate neonatal hormonal dependence. These results are consistent with other ontogenic studies, and show that synapse-associated mRNAs are differe ntially regulated, and are coincidental with synaptic ontogeny in thes e areas. They also confirm a sex dimorphism of these mRNAs in adult CT X, and show that these dimorphisms are not present at birth, but rathe r become apparent at adulthood. Their adult expression is determined, at least in part, by the availablity of testosterone substrate to be a romatized in situ to estradiol, and the ability of this estradiol to i nteract with CTX estrogen receptors present at birth. Thus, GAP-43 mRN A and SNAP-25 mRNA appear to be markers for sex-specific synaptic onto geny, and are likely to contribute to both the structural and function al bases of rat brain sex differentiation.